Two-way radio communication systems commonly use either frequency division duplexing (FDD) or time division duplexing (TDD). The frequency division duplexing approach uses well separated frequency bands in order to avoid interference between uplink and downlink transmissions, whereas with the time division duplexing approach, so called TDD, uplink and downlink traffic are transmitted in the same frequency band or adjacent frequency bands, but in different time intervals. A benefit of the time division duplexing mode of operation is that paired well separated frequency bands are not required.
In the Long Term 3G Evolution (LTE) telecommunication technology, bandwidth flexibility and flexibility of spectrum allocation are two of the key features. In order to utilize also unpaired frequency bands it has been decided that at least one time division duplexing mode should be supported. However time based duplexing also imposes challenges that do not emerge in frequency based duplexing.
In order for transmissions not to interfere with each other, the transmissions in adjacent areas need to be synchronized in some sense.
Further, guard periods, or idle periods, during which no signal is transmitted, need to be introduced in the switching between downlink and uplink in TDD mode communication. One reason for this is the need to allow the equipment in the base stations and the mobiles to switch their circuits between transmission mode and receive mode.
Another reason, which puts stronger requirements on the duration of the guard periods under most circumstances, is that when a base station or mobile is switched from transmission mode to receive mode, signals from other base stations or mobiles are still propagating in the air, causing significant interference. Depending on e.g. the cell size and inter-base station distance, the duration of the guard period length needed differs.
As an example, elevated base stations even 80 km away, corresponding to a propagation delay of about 0.27 ms, may give interference above the noise floor after a downlink to uplink switch. With aggregation of downlink transmissions from several base stations even further away, the interference level may be very significant and may influence reception in a negative way.
Thus, in order to avoid interference in time division duplexing mode communication, guard periods are introduced around switches between uplink and downlink. There could potentially also be other reasons for not allowing transmission involving specific mobiles or base stations during certain time periods.
The guard periods may be the same for the whole network, even if it is not necessarily so. Transmissions must not be scheduled to use any part of the guard periods. Thus transmissions are only scheduled for outside the guard periods. The transmission quality of a communication and/or coherence properties of a communication channel between the base station and the user equipment may differ, depending on a plurality of unwanted influence on the signal and the radio propagation conditions. Some non limiting examples of such unwanted influence may be thermal noise and interference and a non limiting example of phenomena that adversely affect the propagation conditions are path loss, signal multi-path, and Doppler spread. Further the accuracy of channel estimation will affect the transmission quality.
A guard period may be created by having no transmissions on some of the data symbols at the end of a sub-frame. A similar truncation may be done at the beginning of a sub-frame. At some stage, however, it will be necessary to remove also some reference symbols, or pilots, used for channel estimation, in order to create sufficient idle periods. This inevitably results in less accurate channel estimates. Redundant information in data symbols or reference symbols that both may be helpful are truncated away. This is a problem in particular for mobile nodes that have a channel that de-correlates fast over time and/or frequency, so that only very recent reference symbols at neighbouring frequencies carry usable information about the channel at the location of the data symbols, and for mobile nodes with poor channel quality or low signal to noise ratio (SNR).